Heretofore there have been a number of different types of preheating apparatus provided for metal such as metal scrap which is to be processed in furnaces for metal manufacture or which is to be used for other purposes.
Previously U.S. Pat. No. 3,721,519 on "Furnace Charging Apparatus", and U.S. Pat. No. 3,813,209 on the "Preheating of Metal Scrap" have been obtained. The apparatus and the functioning thereof as described in these patents have provided very satisfactory metal preheating actions. Both patents teach use of afterburners in the exhaust gases, and U.S. Pat. No. 3,813,209 teaches reducing the fuel burners to a low fire setting after the metal has reached a desired temperature. It naturally is desirable to preheat metal masses as efficiently as possible, and to preheat such metal scrap masses quickly by use of minimum quantities of fuel.
It is well known that when scrap metal is being heated, it can receive, store and transfer heat at a rate based upon its absorptivity. As this absorptivity decreases as the temperature of the metal increases, the time required for raising the heat from a particular point or temperature (dependent upon the material and initial temperatures involved) to a desired temperature about 1/3 higher than this point, is equal to the time required for the first increase in temperature from, for example, ambient temperatures to the particular starting temperature for the latter increase.
For best heat transfer action, it is necessary to maintain a positive temperature differential and as much direct flame impingement on the material as is possible. Hence, it is believed that if extra heat is suplied to the material in the preheater hood or furnace, it will not be absorbed and such extra heat can be sensed leaving the preheater hood or furnace via the exhaust stack and connecting means.
The general object of the general invention is to provide an improved method for preheating metal scrap or the like in a furnace by control of the fuel supply to the furnace, and characterized by control of the air-fuel supply pressure dependent upon the temperature of the gases exhausting from the furnace.
Another object of the invention is to regulate the pressure of a gaseous fuel to be supplied to a furnace or preheater hood to make it equal or substantially equal to the air pressure supplied to such hood, and is also to control, automatically, the air pressure supplied to the hood in relation to the temperature of exhaust gases leaving the hood, whereby if such exhaust gases are at an excessive temperature, the pressure of the air and fuel supplied to the preheater hood or furnace are reduced.
A further object of the invention is to provide for automatically efficiently utilizing fuel supplied to a metal preheater hood or furnace to prevent excessive fuel supply thereto and consequent excessive temperatures and exhaust gases due to a minimum removal of heat transfer to the processed material while in the preheater hood.
The foregoing and other objects and advantages of the invention will be made more apparent as the specification proceeds.